Smokers need to be aware of another cancer risk factor, which 25 percent of them carries within. Researchers at The Institute of Cancer Research in London found a gene defect that increases risk noticeably. Their work was published in the journal Nature Genetics on Sunday.

After comparing the DNA of 11,348 Europeans with lung cancer and 15,861 cancer-free patients, researchers found a particularly strong contention for those with the squamous cell lung cancer type. Squamous cell carcinoma is a non-small cell lung cancer, accounting for 25 to 30 percent of all lung cancers. The defect in gene BRCA2, which is also known for its role in breast cancer, increases the risk of lung cancer so much that smokers who carry it are likely to develop cancer at some point in their lives.

"Our study showed that mutations to two genes, BRCA2 and CHEK2, have a very large effect on lung cancer risk in the context of smoking. Mutated BRCA2 in particular seems to increase risk by around 1.8 times,” said Richard Houlston, the study’s lead author and professor of molecular and population Genetics at The Institute of Cancer Research, in a press release.

The genetic link is an important find because of the high number of smokers it affects. In the United States, more people die from lung cancer than any other type of cancer, according to the Centers for Disease Control and Prevention.

"Smokers in general have nearly a 15 percent chance of developing lung cancer, far higher than in non-smokers. Our results show that some smokers with BRCA2 mutations are at an enormous risk of lung cancer — somewhere in the region of 25 percent over their lifetime,” Houlston said.

The BRCA2 defect has also been found to increase the risk of breast cancer, ovarian cancer, and other types of cancers. There are already drugs developed that are specifically designed to be effective in treating cancers with BRCA2 mutations. Finding the gene’s connection to lung cancer could mean a door to treatment has been unlocked for 25 percent of the lung cancer population. They could benefit from the PARP inhibitors that work effectively for other cancers with the BRCA2 gene defects.

"Lung cancer claims more than a million lives a year worldwide and is by far the biggest cancer killer in the UK. We know that the single biggest thing we can do to reduce death rates is to persuade people not to smoke, and our new findings make plain that this is even more critical in people with an underlying genetic risk," said Houlston.

As the leading cancer killer for both men and women in the United States, which accounts for 27 percent of all cancer deaths, understanding cancer is key. Researchers also found a connection between squamous cell lung cancer and a defect in another gene, CHEK2, which normally stops cells from dividing after they’ve suffered DNA damage.

"These findings indicate that around a quarter of smokers with a specific defect in their BRCA2 gene will develop lung cancer — a disease which is almost invariably fatal. All smokers are taking a considerable risk with their health, regardless of their genetic profile, but the odds are stacked even more heavily against those with this genetic defect who smoke," said Paul Workman, deputy chief executive of The Institute of Cancer Research, in the release.